Signal generators that can be used to produce sinusoidal oscillations are normally referred to as oscillators. In the case of LC oscillators, the frequency is governed by a resonant circuit with an inductance and a capacitance. The simplest method for producing a sinusoidal oscillation is to use an amplifier to compensate for attenuation of an LC resonant circuit.
The fundamental configuration of an oscillator such as this is described, for example, on pages 458 et seq. of a reference by Tietze, Schenk: entitled xe2x80x9cHaibleiter-Schaltungstechnikxe2x80x9d, 10th Edition 1993.
In order to achieve higher power levels and better efficiencies, oscillators are normally configured in the form of push-pull oscillators, in which two cross-coupled transistors are provided for attenuation compensation, in which case the cross-coupling may, for example, be conductive, capacitive, inductive or transformer positive feedback.
In order to make it possible to produce variable frequencies, it is also normal to configure the integrated capacitance in the LC resonant circuit to be controllable, for example, in the form of a varactor diode.
When oscillator circuits are constructed in the form of integrated circuits, then, with normal manufacturing methods, this necessarily results in process fluctuations that, for example, result in capacitance value tolerances of +/xe2x88x9220%. Discrepancies such as these from the nominal values of the components that are used cause amplitude discrepancies in the output signal from the oscillator, which are undesirable.
It is known for the bias current of the transistors that are provided in the attenuation compensation amplifier, for example MOS field-effect transistors, to be readjusted such that the gradient of the transistors is adapted in a compensating manner. A dependent current source is normally provided for this purpose, although this increases the phase noise of the oscillator. Furthermore, the compensating readjustment of the bias current of the transistors leads to a shift in the operating point, and thus to a poorer drive capability for the transistor.
U.S. Pat. No. 6,118,348 specifies a crystal oscillator circuit. Two or more parallel-connected inverter stages are provided, and are connected to an oscillating crystal on the input side and output side. A control signal generator, which is coupled to the inverter stages, switches between different gain levels. The switching process is carried out with a delay, so that the noise level in the output signal is low.
It is accordingly an object of the invention to provide a compensated oscillator circuit that overcomes the above-mentioned disadvantages of the prior art devices of this general type, which compensates for manufacturing tolerances from the nominal values of the components used, and discrepancies resulting from these tolerances in the amplitude of the output signal, while the oscillator circuit at the same time has good phase noise characteristics.
With the foregoing and other objects in view there is provided, in accordance with the invention, a compensated oscillator circuit. The compensated oscillator circuit contains a supply potential connection, a resonant circuit, at least two attenuation compensation amplifiers coupled switchably to the resonant circuit to compensate for attenuation, and switches. In each case one of the switches is coupled with in each case one of the attenuation compensation amplifiers for forming switchable current paths between the resonant circuit and the supply potential connection. Currents sources are connected to and feed the attenuation compensation amplifiers. One of the current sources is disposed in each of the switchable current paths.
The attenuation compensation amplifiers which can be switched on and off separately from one another allow both the bias current of the amplifiers and the channel width to channel length ratio of the entire oscillator circuit, and hence the gradient of the gain, to be varied and thus to achieve an oscillator output signal with a constant amplitude despite tolerance-dependent discrepancies in the component values from their nominal values. Since it is not only the bias current of the transistors that is changed, this reduces the dependency of both the supply current and the operating point for the transistors on the actual component values. In this case, the attenuation compensation amplifiers are effectively connected into the oscillator circuit, or are disconnected from it, independently of one another, in order to achieve the desired oscillation amplitude at the output of the circuit, for example, in order to compensate for manufacturing-dependent discrepancies from a desired oscillation amplitude.
The amplitude of the output signal from the oscillator circuit increases as the transistor gradient of the attenuation compensation amplified transistors increases. The gradient is in this case approximately proportional to the square root of the product of the bias current and of the channel width to channel length ratio of the transistors.
Overall, the present invention makes it possible to considerably reduce discrepancies from the ideal operating point of the oscillator amplifier, that is to say of the attenuation compensation amplifier. Thus, overall, this considerably reduces reductions in performance resulting in discrepancies of the components used from their nominal values.
According to the present invention, the current paths each have a current source for feeding the attenuation compensation amplifiers.
By way of example, switchable current sources may be used, which are each provided in one current path with in each case one attenuation compensation amplifier, in which case the attenuation compensation amplifiers may be permanently connected to the common resonant circuit.
In one preferred embodiment of the present invention, the switches each have a control connection, which is connected to a drive circuit.
The drive circuit therefore makes it possible in a simple manner to select a specific combination of attenuation compensation amplifiers, in order in this way to set the desired overall gradient for the attenuation compensation for the oscillator and, in the end, thus to achieve the desired oscillator signal amplitude.
In a further preferred embodiment of the present invention, a control loop is formed, with amplitude value detection, and is connected on the input side to the resonant circuit and on the output side to the drive circuit.
The formation of a control loop allows automatic compensation for manufacturing-dependent component tolerances by measurement of the amplitude and by switching appropriate attenuation compensation amplifiers on and off in a compensating manner.
In a further preferred embodiment of the present invention, the current paths with the attenuation compensation amplifiers are connected in parallel with one another to the resonant circuit.
In a further preferred embodiment of the present invention, the attenuation compensation amplifiers each have two cross-coupled transistors.
When using field-effect transistors, the cross coupling of the transistors can be achieved by connection of in each case one gate connection of a transistor in the transistor pair crossed over to in each case one drain connection of a further transistor in the transistor pair.
The coupling may in this case be directly conductive, capacitive, or by transformer. The source connections of a transistor pair are connected directly to one another at a source node, and are connected to a current source that can be switched on and off. This results in a switchable current path for feeding the attenuation compensation amplifiers.
In a further preferred embodiment of the present invention, the transistors in the attenuation compensation amplifiers are MOSFET transistors, which have the same channel width to channel length ratio in pairs, with the channel width to channel length ratio of the attenuation compensation amplifiers being graduated in binary steps with respect to one another.
The binary graduation of the channel width to channel length ratios that influence the gradient allows a good compensation capability for manufacturing-dependent component tolerances, with a relatively small requirement for components and surface area.
Depending on the field of use for the application of the oscillator circuit, other graduations of the transistor ratios in the attenuation compensation amplifiers with respect to one another may, of course, also be worthwhile.
In a further preferred embodiment of the present invention, the switches are digitally driven transistor switches. Transistor switches based on CMOS or BiCMOS semiconductor technology can be implemented in a simple manner and, furthermore, can be driven easily.
In a further preferred embodiment of the present invention, the resonant circuit has a control input for controlling the oscillation frequency. The resonant circuit is normally in the form of an LC resonant circuit, and in this case the inductance is preferably fixed while the capacitance is controllable, for example, in the form of a varactor that can be driven by a control voltage.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a compensated oscillator circuit, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.